Shale oil extraction process



REFERENEE SEAREE'E WW3? Oct. 28, 1969 H. A. DEANS ET AL 1 3,474,863

SHALE OIL EXTRACTION PROCESS Filed July 28, 1967 HEATER HEAT *2 A EXCHANGE-IR PRODUCED 1 GAS A: SEPARATOR PRODUCED r m 23 on.

FlSCHER ASSAY is F BENZENE,

SUPERCRITICAL w w A A u A 3 l4 TOLUENE, SUBSTANTIALLY 9 SUPERCRITICAL U I0 9-- E 8 TOLUENE, g SUBCRITICAL 6 LIJ 20 so I00 140 I80 TIME lN DAYS INVENTORS:

2 HARRY A. DEANS MICHAEL PRATS THEIR ATTORNEY nited US. Cl. 166-266 6 Claims ABSTRACT OF THE DISCLOSURE Shale oil is produced by circulating a volatile normally liquid oil solvent through a permeable fragmented zone within a subterranean oil shale formation under supercritical conditions of temperature and pressure.

BACKGROUND OF THE INVENTION Field of the invention The invention relates to a process for extracting shale oil from a subterranean oil shale formation; more particularly, it relates to a process for recovering oil from a permeable fragmented shale oil formation by circulating an oil solvent under supercritical conditions, through the permeable fractured zone or fnagmented oil shale.

Description of the prior art It has been found that the rate at which the kerogen in oil shale is fluidized by contact with a hot normally liquid oil solvent is much greater when the solvent is under critical conditions of temperature and pressure. It is becoming increasingly evident that this can be an economically attractive process for producing shale oil from an oil shale formation.

Kerogen is a bituminous material occurring in oil shale formations and yielding shale oil when heated. A big drawback to the retorting of oil shale to recover shale oil is the need to remove and dispose a substantial amount of the shale after it has been retorted. In view thereof, in situ retorting has found much favor in recent years as a method of extracting shale oil particularly from subterranean oil shale formations. One such method is to create a large, permeable zone within the oil shale formation by creating a rubbled zone by an explosion within the formation, e.g., by utilizing high energy explosives such as nuclear bombs. One or more access wells are then drilled into the fragmented zone and communication is established between the permeable zone and openings into the wells. Hot fluids are then injected, usually to start an in situ combustion heating process which causes the kerogen to become fluidized. The shale oil is then recovered from a production well by conventional means.

SUMMARY OF THE INVENTION It is an object of this invention to produce oil from a subterranean oil shale formation more efliciently than this has been previously accomplished.

The present invention discloses an improved process for producing shale oil from a subterranean oil shale formation. First, a permeable fragmented zone is created within the formation and one or more Wells are drilled into the fragmented zone. Communication is then established between the fragmented zone and at least one opening into a well. A fluid, heated to at least the critical tern perature of a preselected normally liquid, hydrocarbon solvent, is circulated through the permeable fragmented zone. A normally liquid, hydrocarbon solvent is circulated as the hot fluid, along with the hot fluid, or after the hot fluid, and is maintained at a temperature of from approximately its critical temperature to about 900 F.

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under a pressure at least approximately as high as its critical pressure. This causes liquid hydrocarbons to be released from the oil shale formation and entrained in the circulating hot solvent. The liquid hydrocarbons can be recovered from the solvent by procedures such as distillation, or the like.

Other objects of the invention will become apparent from the following description taken in conjunction with the drawing.

BRIEF DESCRIPTION OF THE DRAWING FIGURE 1 shows a partly diagrammatic and partly sectional elevational view of a subterranean oil shale formation; and I FIGURE 2 shows a graphic illustration of the percent weight loss of an oil shale formation over a period of time.

DESCRIPTION OF THE PREFERRED EMBODIMENT FIGURE 1 shows a subterranean oil shale formation 11 and a permeable fragmented zone 12 formed in the formation 11. The term permeable fragmented 'zone refers to a multiply fractured zone in which the number of the fractures and the volumes of the interconnected openings withing the fractures provide a volume of void space in the order of from about five to forty percent of the volume of the zone. Such a zone can be formed by known procedures, such as those disclosed in Patent No. 1,422,204, issued July 11, 1922. Preferably such a zone is formed by fragmenting a relatively large volume of oil shale by the detonation of a high-energy explosive device such as a nuclear bomb, to form a rubble-filled chimney of fragmented oil shale as in FIGURE 1.

Any suitable process can be used to create the perme= able fractured zone. Fluid communication can be established between a pair of points located within the oil shale formation (as, for example, between a pair of well boreholes extending into the formation) by any known means.

Preferably, at least one pair of well boreholes 13 and 14 [are then extended into the permeable fragmented zone 12. Packers 15 and 15' in the casings 16 and 16, respectively, of well boreholes 13 and 14 permit fluid 17 to be circulated through tubings 18 and 18' in casings 16' and 16', respectively, as can be seen in FIGURE 1. Openings 19 in casing 16 permit fluid 17 to horizontally circulate through the tubing 18 of the well borehole 13 through the permeable fragmented zone 12 into the other well borehole 14. In general, the well boreholes 13 and 14 are extended adjacent the bottom of the fragmented zone 12, preferably while the zone 12 is still hot from the nuclear or other explosive energy. Well boreholes 13 and 14 are preferably cased throughout the extent of the fragmented zone 12 and perforated at 19 and 19, respectively, at the depths at which fluid is to be injected and produced.

In the preferred embodiment of the invention, a hot fluid 17 is injected into tubing string 18 and produced through tubing string 18, preferably with packers 15 and 15' set above and below the points of injection and production. The fluid 17 can be a relatively low cost heating fluid such as a fresh, brackish or briny water, which can be softened if necessary by conventional water procedures, or can be the selected normally liquid solvent.

The temperature of the circulating fluid 17 should be increased to the critical temperature of a normally liquid solvent. For example, when water is used as a heating fluid 17 and benzene is used as the solvent, the fluid being circulated is heated in heater 20 to at least 550 F.

The preferred solvents comprise volatile, normally liquid components of shale oil, low molecular weight, predominantly aromatic mixtures of hydrocarbons, such U as relatively low purity refinery by-products of low value, and the like. Such solvents at supercritical conditions have been found to be particularly effective in extracting shale oil hydrocarbons. At substantially supercritical conditions, the pressure of the volatile material exceeds the critical pressure while the temperature is at least approximately as high as the critical temperature. In respect to benzene, these values are 550 F. and 700 p.s.i., and for toluene, they are 600 F. and 615 p.s.i. In respect to mixtures, the values decrease with increases in the average molecular weights of the components.

At the same time the temperature of a circulating hot fluid, such as an aqueous fluid, is being increased to the critical temperature of a selected solvent, such as benzene, the pressure in the permeable zone 12 is preferably in-= creased, if necessary,to provide a pressure at least as high as the critical pressure of the solvent. At this time, the circulating hot fluid 17 is converted, preferably in a gradual manner, from the aqueous fluid to a mixture of aqueous liquid and solvent liquid and then to solvent (i.e., a solvent mixture of the aqueous fluid and the selected solvent) at supercritical conditions. In general, the temperature within the flow path and the pressure within the flow path is increased to a pressure exceeding the critical pressure of the solvent, i.e., exceeding 700 p.s.i. in the case of benzene. Where necessary in order to maintain supercritical conditions within the flow path, the flow of fluid from the production well borehole 14 is throttled and the injection pressure into well borehole 13 is increased to above the critical pressure of the solvent 21 to the extent required to provide an adequate rate of flow at a supercritical pressure.

The hydrocarbon solvent is circulated at supercritical conditions along the same flow path at a given depth in the fragmented zone 12 in oil shale formation 11 until at least a substantial proportion of the oil shale hydrocarbons has been extracted from the rocks within that flow path. The level along which the solvent is circulated is raised or lowered by shortening the tubing strings 18 and 18' and opening perforations 19 and 19 through the casings 16 and 16' at higher or lower levels in order to adjust the level of the points of fluid injection from Well borehole 13 and mixture production from well borehole 14.

Fluid produced from well borehole 14 passes into heat exchanger 22 and then into separator 23 whereby shale oil and gases are separated from the mixture by means well known in the oil extraction art. The remaining fluids, mainly, the selected solvent, are recirculated into heater 20 and production well 13.

In a preferred horizontal displacement operating procedure, as disclosed in FIGURE 1, water can be used to limit the amount of solvent that is to be circulated. In this well arrangement, the permeable zone 12 is filled with water 24 to a depth just below that at which a horizontal flow path is to 'be established. The water 2 4 can be injected at ambient surface temperature. The pressure of the water is then adjusted to attain the supercritical temperature and pressure for the mixture of solvent and water vapor that will be present in the flow path. Where the amount of shale oil hydrocarbon that is released by the explosive energy is sufficient to cause the pressure to be too high when the water level is raised to the selected depth, hydrocarbon should be produced from the permeable zone 12. Where the pressure is too low after the water level 25 of water 24 has the desired elevation, a gas such as air should be injected through well borehole 13 to increase the pressure in the initial stages of the operation. Numeral 26 in the drawing refers to a void space, gas or vapor filled, formed in the upper portion of fragmented zone 12 after the oil. shale formation 11 has been exploded.

The flow path near the top of the permeable zone 12 along which a hot fluid circulates is preferably both heat ed and established as the path of preferred flow for circulating fluid by injecting and producing either heated solvent or steam. The elevation of the flow path can be decreased either incrementally or substantially continuously. Water is produced during the production of the mixture of solvent and aqueous fluid containing entrained oil shale hydrocarbons and the produced water must be replaced, either by recycling and/ or adding makeup water, when the elevation of the circulation path is being kept constant. This replacement of water can be carried out by means well known in the art.

The present process can also be operated by circulating the solvent at supercritical conditions along a generally vertical flow path in the manner disclosed in copending application. Serial No. 632,006, filed April 19, 1967. In that application shale oil is recovered from a subterranean oil shale formation by circulating a fluid heated at a moderate temperature from one point within a permeable zone to another along a generally vertical flow path for a relatively long period of time until a substantial proportion of the organic compounds contained in the oil shale are converted to oil-shale-derived. fluidizable materials.

It has been found that the pyrolysis of oil shale in a fragmented permeable zone within an oil shale formation can be significantly benefited by the presence of hydrogen sulfide, at least at temperatures ranging from about 480 to 710 F. under pressures above 1,000 p.s.i. Accordingly, significant preselected proportions of either hydrogen sulfide alone or a mixture of hydrogen and hydrogen sulfide, can be added to either or both the preselected normally liquid hydrocarbon solvents or the aqueous fluids which are circulated at the substantially supercritical conditions of the instant invention.

FIGURE 2 illustrates graphically the effect of a liquid hydrocarbon solvent, such as benzene, an oil shale at supercritical conditions. The percent weight loss of the oil shale appreciably increases over a relatively short period of time. The dashed-line curve shows the weight loss produced by circulating benzene through a crushed oil shale having a Fischer assay corresponding to a weight loss of about 19.3%. The benzene was circulated at about 550 F. and 700-960 p.s.i.g. (above its critical condition of 552 F. and 700 p.s.i.) for about 45 days. The solidline curves show two toluene extractions of a shale having a Fischer assay corresponding to a 14.2% weight loss. The lower solid-line curve relates to circulating toluene at the subcritical conditions of 550 F. and 350 p.s.i.g. (relative to 600 F. and 615 p.s.i.) for about 108 days after which the conditions were changed to substantially supercritical conditions by increasing the pressure to 800 p.s.i.g. The upper solid-line curve relates to circulating toluene at 550 F. and 800 p.s.i.g. for days.

It can thus be seen that normally liquid hydrocarbon solvent, such as benzene, operating at substantially supercritical conditions, can be very effective in shortening the time required for in situ retorting of a fragmented perme able zone within an oil shale formation. Other solvents with similar characteristics such as toluene, can be used.

We claim:

1. In a process for producing shale oil from a subterranean oil shale formation comprising:

forming a permeable fragmented zone within a subter ranean oil shale formation; extending at least one well borehole into the fragmented zone within said subterranean oil shale formation;

circulating a fluid through said fragmented zone while heating said fluid to at least approximately the critical temperature of a preselected normally liquid hydrocarbon solvent under the critical pressure for said solvent;

circulating said preselected solvent through the per meable zone at a temperature of from approximately its critical temperature to about 900 F,; maintaining said solvent at substantially said supercritical conditions during circulation thereof through said permeable zone;

recovering shale oil hydrocarbons and circulating fluid.

from the underground formation; and

separating the shale oil hydrocarbons from the circu 2. In the process of claim 1 wherein the step of main= taining said normally liquid hydrocarbon solvent at supercritical conditions includes the step of maintaining said solvent at said conditions by throttling the outflow of flow of fluid from said fragmented zone While increasing the injection pressure of the infiowing fluid to above the criti= cal pressure of the said solvent to the extent required to provide an adequate rate of flow of the circulating fluid.

3. In the process of claim 1 wherein at least a pair of xvell boreholes are extended into the fragmented zone and including the step of filling the permeable fragmented zone with water to a depth just below that at which a flow path is to be established between said pair of boreholes thereby limiting the amount of preselected solvent that is required to be circulated,

4'. In the process of claim 3 including the step of injecting a gas into the upper portion of said permeable fragmented zone in order to increase the pressure of said zone in the initial stages of the process,

51 In the process of claim 1 including the step of adding significant proportions of hydrogen sulfide to said nor= mally liquid hydrocarbon solvent which is circulating through said fragmented zone, thereby reducing the over= all time required to entrain a significant portion of the liquid hydrocarbons in the mixture.

6. In the process of claim 1 including the step of adding significant preselected proportions of hydrogen sulfide and hydrogen to said normally liquid hydrocarbon solvent which is circulating through said fragmented zone, thereby reducing the overall time required to entrain a significant portion of the liquid hydrocarbons in the mixture.

References Cited UNITED STATES PATENTS 1,422,204 7/ 1922 Hoover et a1 l6611 3,064,728 11/1962 Gould 166-11 X 3,084,919 4/1963 Slater 166-11 3,241,611 3/1966 Dougan 1661l X 3,284,281 11/1966 Thomas .m 166-11 X 3,285,335 11/1966 Reistle 166-40 X 3,322,194 5/1967 Strubhar -t 1661l 3,342,257 9/1967 Jacobs et al. aaaa 16636 X 3,352,355 11/1967 Putman 16611 3,358,756 12/1967 Vogel a 16611 X STEPHEN J. NOVOSAD, Primary Examiner US. Cl. X R. 

